Sanguinicola inermis Plehn, 1905, a recently introduced fish pathogen in Britain, has caused considerable damage to the carp industry. Two severely affected fish farms in England were included in the study. The incidence of fish infection for both farms was high, being 74-84% in 0+ and 1+ carp. Studies were made on the infection process and details of route of entry, migration and maturation of the worm are presented. An apharyngeate, furcocercous, lophocercous cercaria which develops in the snail Lymnaea (Radix) peregra (Mü11) was shown experimentally to develop into an adult Sanguinicola inermis. Maximum penetration of cercaria was achieved within 30 minutes and fins appeared to be the preferred site. Large numbers of worms were found to remain in the skin after penetration where they continued to develop to maturity, a previously unreported feature of S. inermis infection. The greatest migration to preferred loci occurred at 60 days post infection immediately prior to egg production. The migratory route used was found to be the loose connective tissue as well as the circulatory system. The distribution of mature worms in major blood vessels changed with season. Light microscopical studies were made on the morphology of specimens collected from the two farms and these were identified as S. inermis. Scanning electron microscopy of the surface topography and transmission electron microscopy revealed details of the tegument and provided evidence for the absence of spines. The major developmental features of the worm are described. A marked increase in size occurred up to 36 hours post penetration. Subsequent growth was slower. Egg production by mature worms began 10 weeks post infection at 15°C. Distribution and development of eggs in tissues is described. Egg production continued throughout the year with peaks during the summer months despite the constant environmental conditions. The growth rate of 0+ fry was studied over a period of 16 weeks and heavily infected fish showed stunted growth and poor Specific Growth Rates, Protein Efficiency Ratios and Food Conversion Ratios. Haematological studies showed that infected fish suffered from hypochromic macrocytic anaemia with leucocytosis and an increased Erkhrocyte Sedimentation Rate. Three different phases of infection were recognized. During phase I, the invasive stage, mortality may occur if infection levels are extremely high, but lightly infected fish present as clinically normal. Phase II was found to be the most critical phase since the majority of fish (over 90%) died at this stage. Histopathology revealed the progress of the infection from cercarial invasion to worm migration, maturation, egg production and miracidial hatching. Pathological changes were observed in the skin in phase I. In phase II, the heart, gills and kidneys were severely affected by both mature worms and developmental stages. The infection reached a chronic stage during phase III evidenced by a granulomatous tissue reaction largely in response to residual eggs in tissues. An attempt was made to integrate the phases of infection, development of the worm and pathogenesis in order to elucidate the host-parasite relationships.